1. Field of the Invention
The invention relates to an improved composition suitable for use as a desensitizing solution in treating image-bearing lithographic printing plates, a method for treating such plates employing the improved composition, and a plate treated with the improved composition. The improvement comprises employing a polyoxyalkylene glycol in the old composition to prevent a tacky stage from occur-ring as the treated plate is dried.
2. Description of the Prior Art
Lithographic printing plates on which an image has been exposed and developed, referred to as "imaged" plates, must be treated with a natural or synthetic desensitizing gum before being stored or used. One such synthetic desensitizing gum is described in the U.S. Pat. application by Walter L. Garrett et al., Ser. No. 586,455, filed June 12, 1975, entitled "Method of Treating Image-Bearing Lithographic Plates", which is a continuation-in-part of Ser. No. 565,915, filed Apr. 7, 1975, both of which are now abandoned. Said Ser. No. 586,455 and its teachings are hereinafter referred to as "Garrett I". The gum is applied to the plate in an aqueous solution, which is normally wiped dry before the plate is stored or used. Although the synthetic gum of Garrett I is believed to represent a significant development in the art, it suffers from one disadvantage in that when using gums of the type described in Garrett I, tackiness occurs in the final drying stage of the gummed plates. Attempts to rub the plate dry in this stage can result in scuffing of the polymeric coating, which results in sensitized spots on the plate and later leads to scumming on the printed sheets. Plates treated with the original composition of Garrett I therefore could not readily be stored without a long drying period or, alternatively, forced drying as with a fan. Accordingly, a synthetic desensitizing solution was needed which would retain the desensitizing properties and other advantages of the synthetic gum, but which would not have a tacky drying stage.
It is known in the art to use polyethylene glycol in a fountain solution of a lithographic press to prevent the lithographic plate from picking up lint from the paper as it is printed. However, it is believed it has heretofore not been known to include polyglycols in the plate desensitizing solution, where lint is not a problem.